In the drive for higher integration and operating speeds in LSI devices, the pattern rule is made drastically finer. The rapid advance toward finer pattern rules is grounded on the development of a projection lens with an increased NA, a resist material with improved performance, and exposure light of a shorter wavelength. In particular, the change-over from i-line (365 nm) to shorter wavelength KrF excimer laser (248 nm) brought about a significant innovation, enabling mass-scale production of 0.18 micron rule devices. To the demand for a resist material with a higher resolution and sensitivity, acid-catalyzed chemical amplification positive working resist materials are effective as disclosed in U.S. Pat. No. 4,491,628 and U.S. Pat. No. 5,310,619 (JP-B 2-27660 and JP-A 63-27829). They now become predominant resist materials especially adapted for deep UV lithography.
Resist materials adapted for KrF excimer lasers enjoyed early use on the 0.3 micron process, went through the 0.25 micron rule, and currently entered the mass production phase on the 0.18 micron rule. Engineers have attempted test fabrication on the 0.15 micron rule and started investigation on the 0.13 micron rule, with the trend toward a finer pattern rule being accelerated. A wavelength change-over from KrF to shorter wavelength ArF excimer laser (193 nm) is expected to enable miniaturization of the design rule to 0.13 μm or less. Since conventionally used novolac resins and polyvinylphenol resins have very strong absorption in proximity to 193 nm, they cannot be used as the base resin for resists. To ensure transparency and dry etching resistance, some engineers investigated acrylic and alicyclic (typically cycloolefin) resins as disclosed in JP-A 9-73173, JP-A 10-10739, JP-A 9-230595 and WO 97/33198.
With respect to F2 laser (157 nm) which is expected to enable further miniaturization to 0.10 μm or less, more difficulty arises in insuring transparency because it was found that acrylic resins are not transmissive to light at all and those cycloolefin resins having carbonyl bonds have strong absorption. Polymers having benzene rings have a somewhat improved transmittance in proximity to 160 nm wavelength, which is far below the practically acceptable level. It was found that in single layer resists, reducing carbon-to-carbon double bonds as typified by benzene rings and carbon-to-oxygen double bonds as typified by carbonyl groups is essential for insuring a light transmittance (see International Work Shop 157 nm Lithography MIT-LL, Boston, Mass., May 5, 1999). It was reported in J. Vac. Sci. Technol., B17(6), November/December 1999 that introduction of fluorine is effective to improve transmittance. A number of fluorinated polymers for resist compositions were proposed (see J. Photopolymer Sci. and Technol., Vol. 13, No. 4 (2000), pp. 657-664 and pp. 451-458). However, the transmittance of these polymers does not reach that of polyhydroxystyrene and derivatives thereof adapted for KrF exposure and poly(meth)acrylic derivatives and polycycloolefin derivatives adapted for ArF exposure.
As is known in the art, the bi-layer resist method is advantageous in forming a high-aspect ratio pattern on a stepped substrate. It is also known that in order that a bi-layer resist film be developable with a common alkaline developer, high molecular weight silicone compounds having hydrophilic groups such as hydroxyl and carboxyl groups must be used.
Among silicone base chemically amplified positive resist compositions, recently proposed were those compositions for KrF exposure comprising a base resin in the form of polyhydroxybenzylsilsesquioxane, which is a stable alkali-soluble silicone polymer, in which some phenolic hydroxyl groups are blocked with t-BOC groups, in combination with a photoacid generator (see JP-A 6-118651 and SPIE vol. 1925 (1993), p. 377). Positive resist compositions comprising as a base a silsesquioxane of the type in which cyclohexylcarboxylic acid is substituted with an acid labile group were proposed for ArF exposure (see JP-A 10-324748, JP-A 11-302382, and SPIE vol. 3333-07 (1998), p. 62). Also proposed were silicone-containing polymers prepared from silicon-containing acrylic monomers (see JP-A 9-110938, J. Photopolymer Sci. and Technol., Vol. 9, No. 3 (1996), pp. 435-446).
The silicon-containing polymer of the acrylic pendant type has the drawback that its resistance to dry etching with oxygen plasma is weak as compared with the silsesquioxane polymer. A low silicon content and a different polymer main skeleton account for this weak dry etching resistance. The siloxane pendant type polymer has the other drawback of poor developer wetting that it is likely to repel a developer solution. SPIE vol. 3678, pp. 214, 241 and 562 describes a polymer derived from a monomer of the trisilane or tetrasilane pendant type having an increased silicon content and a silicon-containing substituent which can be eliminated with acid. However, since di- and polysilane compounds exhibit strong absorption at a wavelength of less than 200 nm, an increased introduction of such silanes undesirably leads to a lower transmittance. Besides, an attempt of introducing silicon into acid labile groups is reported in SPIE vol. 3678, p. 420. Because of a low acid elimination ability, there are drawbacks including low environmental stability and a T-top profile.
The applicant/assignee proposed in U.S. Pat. No. 6,492,089 (based on Japanese Patent Application No. 11-342380), an acid labile group in the form of a cyclic hydrocarbon group having silicon introduced therein. This polymer has the advantages of an improved acid elimination ability and prevention of T-top profiling and allows dry etching resistance to be enhanced by introducing two or more silicon atoms in a single cyclic hydrocarbon group. A concern about a loss of transmittance to ArF exposure is eliminated.
The serious drawback of acrylic pendant type silicon-containing polymers is a low glass transition temperature (Tg). Since chemically amplified resist compositions are designed such that elimination reaction of acid labile groups is induced by post-exposure baking (PEB), the polymer should have a Tg equal to or above the baking temperature. The PEB temperature is usually 80 to 150° C., and a Tg of about 150° C. is necessary. If PEB is performed above the Tg of the polymer, the acid diffuses through the thermally flowing polymer, losing control on pattern dimensions after development.
Another drawback of the acrylic pendant type silicon-containing polymers is that when observed under a critical dimension measurement SEM, line dimensions are reduced during measurement. This is because ester groups are cleaved by irradiation of electron beams and the resulting ester end moieties volatilize off. A further drawback is slimming of a resist film upon exposure to a F2 laser. This is also because volume shrinkage occurs as a result of ester groups being cleaved by irradiation of VUV laser beams. It is pointed out that when a polymer having silicone-pendant ester groups is irradiated with VUV laser beams, silicone compounds can volatilize out of the polymer. The volatilized silicone compounds will deposit on the surface of a projection lens, leading to a transmittance decline. It is reported that deposits of silicone compounds are difficult to remove as compared with hydrocarbon deposits.
We then presumed that the above-discussed problem can be solved by a resist composition comprising a polymer obtained by polymerizing a vinylsilane or allylsilane derivative having silicone pendent from unsaturated bonds for polymerization, rather than the ester pendant.
In the prior art, for a monomer having a silicon pendant from an unsaturated bond such as vinylsilane, neither radical polymerization of the monomer alone nor copolymerization of the monomer with norbornene or acrylic monomers is possible.
The copolymerization of allylsilane with maleimide is reported in Proc. SPIE, Vol. 1262, p. 110 (1990), and the copolymerization thereof with maleic anhydride reported in Proc. Society of Plastics Engineers Inc., Mid-Hudson Section, 1991, 1466, 227. Unfortunately, these copolymers are less resistant to etching because allylsilane contains only one silicon.
Aiming to increase the silicon content, we synthesized vinyl and allyl compounds having cyclic siloxane of at least three silicon atoms on a side chain and copolymerized them with maleic anhydride to form polymers which had enhanced etching resistance (see Japanese Patent Application No. 2001-056536 and No. 2001-056543 or U.S. Ser. No. 10/085,935). As opposed to chain-like siloxanes generally having low Tg and allowing expansion of acid diffusion, cyclic siloxanes have the advantages that Tg decline is limited and acid diffusion is controlled minimum. It is well known in Proc. SPIE, Vol. 1466, p. 520 (1991) that vinylsilane and allylsilane copolymerize with sulfur dioxide. However, polymers of sulfur dioxide are used in electron beam resists of the backbone decomposition type and characterized in that they are readily cleaved upon exposure to light, plasma or electron beams. A phenomenon that lines are slimmed upon exposure to electron beams during SEM observation was ascertained. It was also found that the sulfur dioxide polymers are less resistant to oxygen plasma etching.